1. Field of the Invention
The present invention relates to a bias circuit and more particularly to a dc bias circuit of an amplifier for a microwave circuit.
Generally, various kinds of amplifiers for microwave circuits have been proposed and used. Most of these amplifiers are designed to be used not over a wide bandwidth of the microwave frequency range, but over a relatively narrow bandwidth of the microwave frequency range. However, in recent years, the demand for amplifiers which are suitable to be used over a wide microwave frequency range has increased greatly in, for example, microwave communication systems having multiple communication channels. In such communication systems, it is preferable to employ an amplifier therein which is operable with respect to each of the multiple communication channels. Consequently, it is desirable to provide an amplifier which can be used over a wide microwave frequency range.
2. Description of the Prior Art
The construction of a wide bandwidth amplifier from a pair of amplifier modules and directional quadrature couplers associated in known, as will be explained hereinafter. Specifically, this type of wide bandwidth amplifier of the prior art is comprised of a pair of amplifier modules, a directional quadrature input coupler which is located at the respective inputs of the amplifier modules, a directional quadrature output coupler which is located at the respective outputs of the amplifier modules and a dc bias circuit. The dc bias circuit supplies a dc bias current to the amplifier modules. The directional quadrature input coupler is very useful for obtaining an optimum input impedance for the pair of amplifier modules in the wide bandwidth amplifier. However, this wide bandwidth amplifier requires a very complicated wide bandwidth dc bias circuit. Therefore, this kind of wide bandwidth amplifier is very high in cost. Accordingly, a substantially wide bandwidth dc bias circuit, disclosed hereinafter, which is relatively low in cost, is conventionally utilized as the dc bias circuit. This conventional wide bandwidth dc bias circuit is comprised of four bias chokes located at respective inputs and outputs of the amplifier modules. However, this conventional wide bandwidth dc bias circuit has the following two defects. Firstly, it is considerably difficult to design suitable rf chokes for forming the wide bandwidth amplifier. Secondly, it is impossible to obtain a gain-frequency characteristic which is substantially the same as that obtained by using the previously mentioned very complicated wide bandwidth dc bias circuit.